Migrate example Android build to Flutter Plugin DSL and JDK 17

[Giga-Maisuradze]

Migrate example Android build to Flutter Plugin DSL and JDK 17

Update the example app's Gradle setup to the modern declarative
Plugin DSL and bump the toolchain for AGP 8.7 compatibility:

• settings.gradle: replace legacy app_plugin_loader.gradle apply with
  pluginManagement/plugins blocks (AGP 8.7.3, Kotlin 1.9.24)
• app/build.gradle: use plugins {} block, drop imperative apply/flutterRoot
• build.gradle: remove obsolete buildscript classpath block
• pin sourceCompatibility/targetCompatibility/jvmTarget to Java 17
  (in both the plugin and example modules)
• bump Gradle wrapper 8.5 -> 8.14
• add android.builtInKotlin/newDsl flags from the Flutter migrator